The basic pattern of the tetrapod limbs of amphibians is preserved in reptiles: a single proximal bone is followed dis-
tally by paired bones. In the fore limb is the humerus followed by the radius and ulna. In the hind limb is the femur followed by the tibia and fibula. The wrist and hand are formed from the carpal and metacarpal bones, and the ankle and foot are formed from the tarsals and metatarsals, five or fewer digits bearing horny claws distal to both wrist and ankle. Reptile orders show enormous variation in the precise form and arrangement of these basic elements and in their behavioral deployment. In squamates these elements are abandoned in favor of serpentine locomotion, which requires an elongate body and therefore an increased number of vertebrae, more than 400 in some snakes. Serpentine locomotion depends on friction between the animal and the substrate, which in some animals is accomplished by pressing the posterior edges of the belly scales against stationary objects so that Newton's third law (for every action there is an equal and opposite reaction) can operate. Some lizards have lost their limbs and use serpentine movement. Others with perfectly fine legs will, in bunch grass habitats, fold the limbs against the body and exhibit facultative serpentine movement, presumably because this type of movement produces faster escape behavior than does ordinary running in tangled vegetation. The twisting and bending of the trunk required in serpentine movement enhance the danger of vertebral dislocation. This selective pressure has been answered by the development of an extra pair of contact points between adjacent vertebrae in snakes, bringing the total number of articular points to five per vertebra. The result is that each vertebra is essentially locked to the next and resists dislocating forces arising from roll, pitch, and yaw.
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